ddddddddddd Thanks for sending us your Unity project files! I was able to confirm the problem, however, the package even included unnecessary files such as the Library folder, so if you are asked to send us a minimal Unity project again in the future, please remove them before sending it to us.
It seems that the skeleton that is causing the error is using a clipping attachment, so if you enable the Use single submesh option of the SkeletonAnimation component in this state and do an alpha animation, it seems that the runtime tries to add a submesh and causes the error as it is prohibited by the option. In the case of your skeleton, it looked like you could adjust the mesh hull without using clipping so that the hair would not stick out of the hood, so you might want to try removing the clipping attachment. In fact, clipping in the Spine Runtimes is implemented using the CPU can be a very expensive operation, so it is better to avoid using it as much as possible in cases where it can be avoided.
Unfortunately, I am not sure why adding a submesh is necessary when using clipping and alpha animation at the same time, so perhaps Harald will explain this when he returns.
